1. Field of the Invention
The present invention relates to a vector network analyzer (VNA). More particularly, the present invention relates to reduction of size, power consumption and weight of components used in the VNA. The present invention also more particularly relates to configuration of the VNA to enable improved operation the presence of external signals.
2. Description of the Related Art
Recently, use of wireless networks for telephone or computer local area network (LAN) communications are being adopted worldwide in the 0-3 GHz frequency range. For instance, cellular telephones in the United States operate in the 800 MHz range. Further, in the United States, the Federal Communications Commission (FCC) has allocated five frequency bands 1850-1990, 2110-2150 and 2160-2200 MHz for emerging technologies which includes personal communications services (PCS). PCS includes sophisticated pocket telephones which will work anywhere--indoors and outdoors, at home or at work or while traveling. PCS also includes wireless LANs.
The wireless communications networks such as cellular telephones or PCS require remotely located antennas or "cells" to provide services to individual users. To reduce service costs for the remote antennas, it is desirable to test the remote antenna and its associated microwave components at its remote location and locate and repair portions which fail. Unfortunately, test devices which make both amplitude and phase measurements, referred to as vector network analyzers (VNAs), which can be easily hand carried by one person are not currently available.
VNAs enable a user to easily identify a fault and to measure the distance from the test device to a fault or discontinuity. A fault may result from environmental conditions such as corrosion of a connector, or from faulty installation or repair, for example, where a coaxial cable is punctured by an installer. Test measurements are first typically made using a VNA operating in the frequency domain to determine whether a fault exists as indicated by an undesirable standing wave ratio (SWR). If a fault is discovered, an analysis of the results derived from the frequency domain measurement is made in the time domain to locate the position of the fault.
A VNA also enables calibration to extend the test port connection to the end of a cable connected to the test port. By using a VNA to extend the test port to the end of the cable, errors in the cable will not be taken into account when measuring a device through the cable. Scaler devices which measure only amplitude do not enable extension of the test port to the end of a cable.
Because a remote antenna may be constantly providing signals for communications devices once it is installed, it is desirable that the remote antenna not be disabled for testing. By measuring both phase and amplitude, a VNA provides the ability to distinguish extraneous signals, enabling tests to be performed with the remote antenna active. Scaler devices cannot distinguish the extraneous signals, requiring that scaler tests be performed with the remote antenna disabled.
As mentioned above, VNAs are large and not easily transportable. Current VNAs have a housing greater than two feet on a side and with a power supply may weigh 50 pounds or more. The VNAs are typically transported by truck to the remote antenna sight and carried by two people to the remote antenna. With remote antennas located on top of towers, transportation of the large VNA proves especially difficult.
The large size and weight of current VNAs is due to the size of synthesizers required to produce the test signal for the VNAs. To provide a test signal over the broad frequency spectrum, such as 0-3 GHz, current analyzers mix a signal from a narrower frequency band synthesizer. However, such mixing requires the narrower bandwidth synthesizer to operate at a much higher frequency than the maximum test signal frequency desired. Increasing the frequency of the narrowband synthesizer is undesirable because it requires the size, weight and power consumption of the narrowband synthesizer to increase.
FIG. 1 shows prior art circuitry for a downconverter used in a VNA which provides a 0-1 GHz test signal. As shown, the downconverter utilizes a 2.5-3.5 GHz narrowband synthesizer to obtain the 0-1 GHz test signal. FIG. 1 illustrates that to obtain a given test signal frequency range, the required synthesizer must provide a maximum frequency greater than three times the maximum frequency of the given test signal to avoid spurious mixing products in the signal.
FIG. 2 shows prior art test signal generation circuitry for an analyzer configured to provide a 0-3 GHz test signal. As shown, to produce the 0-3 GHz test signal, the circuitry of FIG. 2 requires synthesizers or phase-locked loops (PLLs) 200, 210 and 220 all operating at 10 GHz or higher, a frequency greater than three times the maximum required test signal frequency of 3 GHz. The output signal from the 10-13 GHz oscillator 202 of synthesizer 200 is mixed with a signal from 10 GHz oscillator 212 of synthesizer 210 in mixer 230 to obtain the 0-3 GHz test signal similar to the circuitry of FIG. 1. The additional oscillator 222 in synthesizer 220 operating at 10,001 GHz has its output mixed in mixer 232 with the output of synthesizer 202 to provide a 0-3 GHz+1 MHz LO signal used to produce an IF signal for the VNA.
Synthesizers, or PLLs 200, 210 and 220 each include a phase detector with a first input receiving a reference signal from a 1 MHz oscillator 234. A second input of each phase detector is connected to its respective oscillator 202, 212 and 222 through a divide-by-N frequency divider, where N is set to convert the signal from its respective oscillator to a 1 MHz signal. The output of the phase detector is provided through an amplifier and low pass filter back to the voltage control input of its respective oscillator.
To provide incident and reflected test signals, the 0-3 GHz signal from mixer 230 is provided through an amplifier and filter 236 to the input of a splitter 238. Splitter 238 has a first output branch providing a test signal through coupler 240 to test port 242. The coupler 240 is connected to provide reflected signals from the test port 242 to a first input of mixer 244. The second output of the splitter 238 provides an incident signal to a first input of mixer 246. Second inputs of mixers 244 and 246 receive the LO signal from the output of mixer 232 through an amplifier and filter 248. The outputs of mixers 244 and 246 provide respective reflected and incident 1 MHz IF signals to a digital signal processor (DSP) 250. The DSP enables determination of both amplitude and phase characteristics of the test signal reflected from a device connected to test port 242.